In general, if a multi-component pigment shows an x-ray diffraction pattern that differs from the x-ray diffraction pattern of a physical mixture of its components, the pigment is called a solid solution. There are mainly two types of clearly-defined solid solutions, a "guest-host" type solid solution and a "solid compound" type solid solution.
A "guest-host" solid solution is a solid solution wherein the x-ray diffraction pattern of the solid solution is substantially identical to the x-ray diffraction pattern of one of the components, called the "host". The host component is said to accept the other component, the "guest", into its crystal lattice.
Solid compounds are solid solutions wherein the two components associate with each other to produce an x-ray diffraction pattern which is different from the x-ray diffraction pattern of any component or a physical mixture of the components.
Solid solutions and solid compounds of quinacridones are known in the art for their pigmentary properties. Quinacridone solid solutions and solid compounds are described in U.S. Pat. No. 3,160,510.
Solid solutions containing two or more of the diketo-pyrrolo[3,4-c]-pyrrole (DPP) series of pigments and their pigmentary properties are also known in the art and are generally disclosed in U.S. Pat. No. 4,783,540.
Solid solutions containing a DPP-type pigment and a pigment of another class, for example, a quinacridone or quinacridonequinone, are described in U.S. Pat. No. 4,810,304.
The present invention is based on the discovery that unsubstituted DPP and dichloro DPP form a binary solid compound within a narrow weight ratio range of from about 35 to 45 weight-percent of unsubstituted DPP and about 55 to 65 weight-percent of dichloro DPP. The present invention is further based on the discovery that the binary unsubstituted DPP/dichloro DPP solid compound readily accepts a disubstituted quinacridone pigment into its crystal lattice to form a ternary "guest-host" solid solution wherein the binary solid compound is the host. 2,9-Dichloroquinacridone, 2,9-dimethylquinacridone and 2,9-difluoroquinacridone are especially useful disubstituted quinacridone pigments which form ternary solid solutions. The binary and ternary solid solution pigments are particularly useful in both solvent and waterborne paint systems.
The inventive binary and ternary solid solutions are suitable as replacements for the perylene pigment, perylene tetracarboxylic acid dianhydride, in paint systems. This is significant because the perylene tetracarboxylic acid dianhydride is unstable in waterborne paint applications, while the present solid solutions are suitable for use in waterborne applications. Thus, the present solid solutions fill a void caused by the unsuitability of the perylene pigment in waterborne paint applications.